What causes large submarine landslides on low gradient (

Submarine landslides can cause damaging tsunamis, the height of which scales up with the volume of the displaced mass. The largest underwater landslides are far bigger than any landslides on land, and these submarine mega-slides tend to occur on open continental slopes with remarkably low gradients of less than 2°. For geohazard assessments it is essential to understand what preconditions and triggers slope failure on such low gradients. Previous work has suggested that generation of high excess pore pressure due to rapid sediment deposition plays a key role in such failures. However, submarine slope failure also occurs where sedimentation rates are low (<0.15 m/ky), such as off north-west Africa. We use a fully coupled stress and fluid flow finite element model to test whether such low sedimentation rates can generate sufficient excess pore pressures to cause failure of a 2° slope. The sensitivity of overpressure generation and slope stability is assessed with respect to different sedimentation rates and patterns, sediment consolidation properties and stratigraphic layer configurations. The simulations show that in general it is difficult to generate significant excess pore pressure if sediment accumulation is slow and the only pressure source. However, we identify a sediment compression behavior that can lead to submarine landslides in locations worldwide. Our results imply that compressibility is an important factor for the stability of low gradient continental slopes

Direct monitoring of active geohazards: emerging geophysical tools for deep-water assessments

Seafloor networks of cables, pipelines, and other infrastructure underpin our daily lives, providing communication links, information, and energy supplies. Despite their global importance, these networks are vulnerable to damage by a number of natural seafloor hazards, including landslides, turbidity currents, fluid flow, and scour. Conventional geophysical techniques, such as high-resolution reflection seismic and side-scan sonar, are commonly employed in geohazard assessments. These conventional tools provide essential information for route planning and design; however, such surveys provide only indirect evidence of past processes and do not observe or measure the geohazard itself. As such, many numerical-based impact models lack field-scale calibration, and much uncertainty exists about the triggers, nature, and frequency of deep-water geohazards. Recent advances in technology now enable a step change in their understanding through direct monitoring. We outline some emerging monitoring tools and how they can quantify key parameters for deepwater geohazard assessment. Repeat seafloor surveys in dynamic areas show that solely relying on evidence from past deposits can lead to an under-representation of the geohazard events. Acoustic Doppler current profiling provides new insights into the structure of turbidity currents, whereas instrumented mobile sensors record the nature of movement at the base of those flows for the first time. Existing and bespoke cabled networks enable high bandwidth, low power, and distributed measurements of parameters such as strain across large areas of seafloor. These techniques provide valuable new measurements that will improve geohazard assessments and should be deployed in a complementary manner alongside conventional geophysical tools

New insights into landslide processes around volcanic islands from Remotely Operated Vehicle (ROV) observations offshore Montserrat

Submarine landslide deposits have been mapped around many volcanic islands, but interpretations of their structure, composition, and emplacement are hindered by the challenges of investigating deposits directly. Here we report on detailed observations of four landslide deposits around Montserrat collected by Remotely Operated Vehicles, integrating direct imagery and sampling with sediment core and geophysical data. These complementary approaches enable a more comprehensive view of large-scale mass-wasting processes around island-arc volcanoes than has been achievable previously. The most recent landslide occurred at 11.5–14 ka (Deposit 1; 1.7 km3) and formed a radially spreading hummocky deposit that is morphologically similar to many subaerial debris-avalanche deposits. Hummocks comprise angular lava and hydrothermally altered fragments, implying a deep-seated, central subaerial collapse, inferred to have removed a major proportion of lavas from an eruptive period that now has little representation in the subaerial volcanic record. A larger landslide (Deposit 2; 10 km3) occurred at ∼130 ka and transported intact fragments of the volcanic edifice, up to 900 m across and over 100 m high. These fragments were rafted within the landslide, and are best exposed near the margins of the deposit. The largest block preserves a primary stratigraphy of subaerial volcanic breccias, of which the lower parts are encased in hemipelagic mud eroded from the seafloor. Landslide deposits south of Montserrat (Deposits 3 and 5) indicate the wide variety of debris-avalanche source lithologies around volcanic islands. Deposit 5 originated on the shallow submerged shelf, rather than the terrestrial volcanic edifice, and is dominated by carbonate debris

Submarine deposits from pumiceous pyroclastic density currents traveling over water: an outstanding example from offshore Montserrat (IODP 340)

Pyroclastic density currents have been observed to both enter the sea, and to travel over water for tens of kilometers. Here, we identified a 1.2-m-thick, stratified pumice lapilli-ash cored at Site U1396 offshore Montserrat (Integrated Ocean Drilling Program [IODP] Expedition 340) as being the first deposit to provide evidence that it was formed by submarine deposition from pumice-rich pyroclastic density currents that traveled above the water surface. The age of the submarine deposit is ca. 4 Ma, and its magma source is similar to those for much younger Soufrière Hills deposits, indicating that the island experienced large-magnitude, subaerial caldera-forming explosive eruptions much earlier than recorded in land deposits. The deposit’s combined sedimentological characteristics are incompatible with deposition from a submarine eruption, pyroclastic fall over water, or a submarine seafloor-hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline. The stratified pumice lapilli-ash unit can be subdivided into at least three depositional units, with the lowermost one being clast supported. The unit contains grains in five separate size modes and has a >12 phi range. Particles are chiefly subrounded pumice clasts, lithic clasts, crystal fragments, and glass shards. Pumice clasts are very poorly segregated from other particle types, and lithic clasts occur throughout the deposit; fine particles are weakly density graded. We interpret the unit to record multiple closely spaced (<2 d) hot pyroclastic density currents that flowed over the ocean, releasing pyroclasts onto the water surface, and settling of the various pyroclasts into the water column. Our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate. The overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column, attesting to complex sedimentation. Slow-settling particles were deposited on the seafloor together with faster-descending particles that were delivered at the water surface by subsequent pyroclastic flows. The final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies. This study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water

Hybrid submarine flows comprising turbidity current and cohesive debris flow: Deposits, theoretical and experimental analyses, and generalized models

Hybrid flows comprising both turbidity current and submarine debris flow are a significant departure from many previous influential models for submarine sediment density flows. Hybrid beds containing cohesive debrite and turbidite are common in distal depositional environments, as shown by detailed observations from more than 20 modern and ancient systems worldwide. Hybrid flows, and cohesive debris flows more generally, are best classified in terms of a continuum of decreasing cohesive debris flow strength. High-strength cohesive debris flows tend to be clast rich and relatively thick, and their deposit extends back to near the site of original slope failure. They are typically confined to higher gradient continental slopes, but may occasionally form megabeds on basin plains, in both cases overlain by a thin turbidite. Intermediate-strength cohesive debris flows typically contain clasts, but their deposits may be <1 or 2 m thick on low-gradient fan fringes, and are encased in turbidite sand and mud. Clasts may be far-traveled, and meter-sized clasts can be rafted long distances across very low gradients if they are less dense than surrounding flow. Low-strength cohesive debris flows generally lack mud clasts, and as cohesive strength decreases further there is a transition into fluid mud layers that do not support sand. Intermediate- and low-strength cohesive debrites are consistently absent in more proximal parts of submarine systems, where faster moving sediment-charged flows are more likely to be turbulent. Intermediate-strength debris flows can run out for long distances on low gradients without hydroplaning. Very low strength cohesive debris flows most likely form through late-stage transformations near the site of debrite deposition, and emplaced gently to avoid mixing with surrounding seawater. The location and geometry of cohesive debrites in hybrid beds are controlled strongly by seafloor morphology and small changes in gradient. Debrites occur as fringes around raised channel-levee ridges, or in the central and lowest parts of basin plains lacking such ridges. Small variations in mud fraction produce profound changes in cohesive strength, flow viscosity, permeability, and the time taken for excess pore pressures to dissipate that span multiple orders of magnitude. Reduction in flow speed can also cause substantial increases in viscosity and yield strength in shear thinning muddy fluids. Small amounts of sediment can dampen or extinguish turbulence, especially as flow decelerates, affecting how sediment is supported or deposited. This ensures that cohesive debris flows and hybrid flows have a rich variety of behaviors